Pediatric respirator

ABSTRACT

A respirator is described using pneumatic elements to generate a variety of operating modes. A gas mixture is passed through a patient connection which is coupled to supply the gas through a patient port to a patient. After passing the patient port the gas is controllably exhausted to atmosphere through an exhaust port in control valve. The control valve regulates the closure of the exhaust port to provide inspiration and expiration control at the patient port. Pneumatic logic elements are combined to provide automatic inspiration and expiration support with different modes such as volume limited or pressure limited and with selection over the duration of the respective breathing phases. Various operating modes are obtained with variable restrictors selectively placed with the pneumatic logic elements.

This application is a continuation in part of a copending patentapplication entitled "Portable Volume Cycle Respirator" filed on Feb.24, 1974 with Ser. No. 445,758, now U.S. Pat. No. 3,910,270, filed bythe same inventor as this application.

This invention relates to respirators. More specifically, this inventionrelates to a time cycled pediatric respirator utilizing pneumatic logicelements.

BACKGROUND OF THE INVENTION

Respirators have become widely used in a large variety of applicationsby hospitals and medical practitioners. Respirators may be used to curediseases such as pulmonary edema, central nervous system depressions,tetanus neonatorum, asphyxia neonatorum, respiratory distress syndromes,hyaline membrane disease as well as many others.

There are many different respirators available some of which aredesigned for specific diseases and others of greater complexity toprovide a multitude of operational modes for different physiologicalrequirements. For example, some respirators provide intermittentpositive pressure breathing (IPPB), either with or without a positiveend expiratory pressure (PEEP). Other respirators may also provide suchrespiratory supports known as intermittent mandatory ventilation (IMV)whereby a patient may be provided with inspirations with relatively longexpirations, or continuous positive airway pressure (CPAP) to provideoxygen or other breathable gas mixtures at a constant pressure on acontinuous, as needed, basis.

Respirators may involve electrical controls such as described in theU.S. Pat. to Wilson, No. 3,191,595 or a pneumatic control as describedin the U.S. Pat. to Hoenig, No. 3,604,415. An intermittent positivepressure breathing respirator is described in the U.S. Pat. to Liston,No. 3,434,471.

The Hoenig patent describes a respirator using three basic pneumaticlogic elements. The flow of breathable gas to the patient is notcontinuous but interrupted by a logic element whose opening and closureis controlled by other logic devices.

The Wilson patent describes a respirator which senses pressuresgenerated in the gas tube leading to the patient mask to initiatecontrol and operation of a main control valve.

Other respirators known to be available in the art provide a patientconnection tube with a continuous flow of gas. The gas flow is permittedto pass through to a control valve which exhausts the gas to ambient ina controllable manner to thereby provide respiratory support at thepatient mask. Such respirators utilize complex and expensive controls togenerate the desired respiratory support.

SUMMARY OF THE INVENTION

In a respirator formed in accordance with the invention, a supply ofbreathable gas is passed to a patient through a connection having apatient port to supply the gas to the patient and a parallel coupledcontrol port. The control port is so located that it permits the supplyof breathable gas to flow continuously to an ambient environment througha control valve to thus allow the patient to draw from the gas as thepatient requires. A pneumatic control circuit is coupled to the controlvalve to regulate the exhaust of the gas from the patient connectionthrough the control port. The control includes pneumatic elements toprovide maximum and minimum pressures at the patient port incorrespondence with desired inspiratory and expiratory paressure levels.A pneumatic timing network provides timing control over inspiratory andexpiratory periods.

A respirator in accordance with the invention can be formed of aconveniently portable device with a simple construction of pneumaticcomponents. A respirator of this invention advantageously employs a fewlogic elements for a wide variety of respiratory support modes such asIPPB, PEEP, CPAP and IMV. A range of inspiratory to expiratory ratiosare conveniently included as well as selection over operationalcharacteristics such as volume limited mode for the delivery of fixedselectable volumes of gas or a pressure limited mode for the delivery ofgas with a predetermined maximum pressure limit. The respirator providesa constant flow of breathable gas which makes the device particularlyuseful for pediatric applications. The respirator consumes a very smallquantity of gas, thus making it particularly suitable in a portableform.

It is, therefore, an object of the invention to provide a respirator ofa relatively simple, inexpensive construction yet capable of providing alarge variety of respiratory support modes which can be convenientlyadjusted to the physiological requirements of the patient to be treated.

BRIEF DESCRIPTION OF DRAWING

These and other objects and advantages of a respirator formed inaccordance with the invention may be understood from the followingdescription of a preferred embodiment described in conjunction with thedrawing wherein

The FIGURE is a schematic representation of a pneumatic respirator inaccordance with the invention.

DETAILED DESCRIPTION OF EMBODIMENT

With reference to the FIGURE, a respirator 10 is shown for controllingand supplying breathable gas through a supply conduit 12 coupled to aninlet port 14 of a wye or T-shaped patient connection 16. The patientbreathes gas provided by a supply 17 from a patient port 18 in theconnection 16 while the gas may continue to flow through a control port20 in the connection 16 for exhaust at a control valve 22.

Gas is continuously made available to the patient port 18 withoutinterruption to provide respiratory support. Inspiration and expirationcontrol are obtained by regulating the exhaust of gas passing throughcontrol port 20 to an exhaust port 24 in control valve 22. Such controlis achieved by controlling the pressure of a control chamber 26 on oneside of a diaphragm 28 operatively coupled to regulate the exhaust ofgas from port 24. The pressure control chamber 26 in turn is regulatedwith a pneumatic control 30.

With the arrangement as illustrated, the patient port 18 is continuouslyprovided with gas so that a patient's breathing demand can be met at anytime without interruption.

The medicinal gas supply 17 provides air or a special mixture of oxygen,depending upon the particular requirements for the patient. A gas source32 for air, and an oxygen source 34 are respectively connected throughcheck-valves 36-36', filters 38-38' and pressure regulators 40-40' toflow meters 42-42'. The flow meters 42-42' are individually adjustableto provide control over the respective rate of the supply of gas. Theoutputs 46 of the flow meters 42 are joined to a common mixing juncture48 coupled to pass the gas mixture through a heated humidifier 50. Theheated humidifier 50 includes a suitable heater 52 and supply of water53 and other control elements as is conventional in the art to impartthe desired amount of water vapor to the medicinal gas. The humidifiedgas is then passed through a water trap 54 formed of a sufficientlylarge tank to enable water droplets to condense and settle out from thegas mixture and provide a suitable breathable medicinal gas mixture.

A conduit network formed of tubes 12, patient connection 16 and tube 58arranges the flow of gas from the water trap 54 past the patient port 18to control valve 22. The tubes 12 and 58 are formed of rigid small boretubes exhibiting very low compliance. In this manner the supply of gasto the patient connection 16 is not exposed to shape changes of thetubes due to pressure variations imposed by the action of control valve22. The tubing 12, 16 and 58 are furthermore kept small to reduce theeffect of gas compression. However, the tubing is selected not too smalllest the flow of gas would be unduly restricted and expiration throughtube 58 would be made more difficult.

A pressure gauge 60 is coupled to tube 58 to sense and register thepressure occurring at the patient port 18. The control valve 22 includesan emergency relief valve 62 used to exhaust gas to ambient when thepressure in tube 58, and thus at the patient port 18, achieves apredetermined dangerously high level.

The passage of gas through port 24 to atmosphere and thus also thepressure at the patient connection 16, are a function of the forceexerted by a diaphragm 28 on the seat 63. This force, in turn, isdetermined by the pressure developed in chamber 26 and area of thediaphragm 28. The pressure in chamber 26 is controlled by pneumaticnetwork 30 coupled to chamber 26 through conduit 64.

Pneumatic logic circuit 30 is formed of a pneumatic oscillator 66 and apressure control pneumatic bistable element 68. The pressure controlbistable element 68 provides regulation for the inspiration andexpiration pressures to be established at the patient connection 16while the oscillator 66 regulates the duration of the inspiratory andexpiratory periods.

The pneumatic logic control circuit 30 uses three bistable elements 68,70 and 72 which are identical though elements 70 and 72 are coupled inoscillator arrangement. The pneumatic bistable elements are shown inschematic fashion since their physical shape may, as is well known inthe art, take many different forms. Thus each element 68, 70 or 72 has acontrol port 74 to receive an input gas pressure with which a three portvalve 76 is switched from a normal position to an actuated position aslong as gas is supplied to control port 74.

Valve 76, as illustrated in the drawing, normally is biased by a spring78 to close an input port 80 and permit the passage of gas betweenanother input port 82 and output port 84. When gas pressure is appliedto control port 74, the valve element 76 is switched against the springbias to close input port 82 and allow gas flow between ports 80 and 84.Another control port 86 is provided to back bias spring 78. In such casethe operation of spring 78 may be given a snap action effect.

Pneumatic power for network 30 is obtained from normally identicalsources 88, 90 which are derived from the air regulator 40 with ashut-off valve 92 interposed between air source 90 and regulator 40. Theshut-off valve 92 enables the removal of pneumatic power from most ofnetwork 30 to establish a particular mode as will be further explained.

The breathing pressure control element is provided with a variablemaximum inspiratory pressure control in the form of a variablerestrictor 94 which establishes the inspiration pressure through inputport 80. An expiration pressure control in the form of a variablerestrictor 96 is coupled through a fixed restrictor 98 to the otherinput port 82 of element 68.

The oscillator 66 is provided with an inspiration time control in theform of a variable restrictor 100 coupled to input port 80 of element70. A similar restrictor for an expiration time control 102 is showncoupled to input 82 of element 70.

In the operation of the respirator 10, before gas is supplied to thebistable logic elements 68, 70 and 72, they are in the position as shownin the drawing due to the bias action of springs 78. When gas is turnedon and valve 92 is open, the gas source 90 coupled to input port 82 ofelement 72 pressurizes line 104 connected to control port 74 of element68, feedback line 106 coupled to control port 74 of element 70 and backbias line 108 connected to control port 86 of element 72.

Pressurization of lines 104 and 106 results in the switching of elements68 and 70, closing their inlet ports 82 and enabling gas flow from theirinput ports 80 to output ports 84. In the oscillator 66, thepressurization of output port 84 of element 70 results in the deliveryof gas to a volume 110 coupled between element 70 and control port 74 ofelement 72. The rise in pressure of volume 110 is, however, initiallyinsufficient to overcome the combined action by spring 78 and the backbias pressure generated at control port 86 in element 72. This back biaspressure is reduced from that available in line 108 by virtue of thepair of series coupled fixed restrictors 112 and 114 bleeding gas toatmosphere.

In the bistable pressure controlling breathing element 68, gas flowsthrough inspiration pressure control 94 to line 64 and to atmospherethrough a fixed restrictor 116. The pressure in line 64 is determined bythe fixed restrictor 116 and the setting of the maximum inspiratorypressure control 94. The gas pressure in line 64 pressurizes chamber 26and thus diaphragm 28 which thereby blocks exhaust port 24 with a forceproportional to the diaphragm area facing chamber 26.

The blockage of exhaust port 24 in turn prevents the escape of medicinalgas whose pressure at the patient port 18 increases. Since thecompliance of the gas tubes 12 and 58 is very low relative to thecompliance of the patient's respiratory system, most of the gas flowsinto the patient's respiratory system to initiate an inspiratory phase.

The volume of the gas supplied to the patient is a function of the flowrate of the gas as determined by the flow meters 42, 42'. The volume isfurther a function of the length of time that the exhaust port 24 incontrol valve 22 is closed as determined by the inspiration time control100. Since the compliance of the tubing is low, and the pressureattained at the patient port 18 is primarily a function of the patientairway resistnce, the volume of gas supplied to the patient iseffectively constant and the respirator operates in a volume limitedmode.

The inspiratory phase continues until the pressure in volume 110 reachesa level sufficiently high to cause bistable element 72 to switch, thusblocking its inlet port 82 and opening inlet port 80. Since port 80 ofelement 72 is open to atmosphere, the gas in lines 104, 106 and 108 aredumped to atmosphere, allowing bistable elements 68, 70 to be reset bytheir respective springs 78 to their normal positions as shown in thedrawing and thus begin the expiratory period.

With bistable pressure control element 68 reset, the gas in line 64 andcontrol chamber 26 in valve 22 is allowed to flow to atmosphere throughrestrictor 116 and series coupled restrictors 98 and 118. The restrictor116 is so selected that the end pressure established in line 64 is afunction of the position of variable restrictor 96. Thus when the latteris set to produce gas flow at some pressure at junction 120, a residualpressure is retained in line 64 and a residual force maintained againstdiaphragm 28 throughout hte expiratory period.

Hence, with variable restrictor 96 set to a desired level, a positiveend expiration pressure (PEEP) is obtained. Such PEEP state has beenshown to be beneficial in different respiratory diseases such as hyalinemembrane disease.

The expiratory period continues for a time period determined by the timeneeded to exhaust the gas in volume 110 through expiration time control102 to atmosphere. When the pressure exerted by the gas in volume 110 atcontrol port 74 of element 72 falls below the force of its spring 78,element 72 is reset to its normal position as shown in the drawing. Thereset action occurs quickly with a snap action as a result of thefeedback obtained along line 108.

Note that the restrictors 112, 114 are so selected that the total resetforce including the force of spring 78 exerted at control port 86 ofelement 72 is less than the maximum setting pressure developed atcontrol port 74 from volume 110 during the inspiratory period.

When the bistable element 72 has been reset, a new cycle commences inthe manner described above. The durations of the inspiratory andexpiratory periods are respectively determined by the settings ofvariable restrictors 100 and 102. The ratios of inspiration toexpiration duration may thus be varied to meet a diverse range ofrequirements.

For example, expiration time control 102 may be adjusted to provideinspirations with relatively long duration expiration, e.g. a ratio ofinspiration to expiration of generally less than about one to five. Thismode is considered an intermittent mandatory ventilation (IMV) which maybe desirable to wean the patient away from dependence upon therespirator. IMV thus gradually (as the ratio is reduced) increases thepatient's ability to sustain his own respirations.

The above description of the operation of the respirator 10 involved aninspiratory period during which the gas pressure at the patient port 18was not sufficient to overcome the exhaust port 24 closing pressureexerted by diaphragm 28. This mode of operation is volume limited whichresults in the application of a constant volume of gas to the patient.However, when the maximum inspiration pressure control 94 is adjusted sothat the gas pressure at the patient port 18 is sufficient to overcomethe diaphragm closure force during each inspiratory period therespirator is operated in a pressure limited mode. When such upperpressure limit occurs, the excess gas at the patient connection 16 isdumped to atmosphere through exhaust port 24.

Another operating mode may be achieved with respirator 10 by closingvalve 92 at the gas supply to effectively disable logic network 30. Insuch case only input port 82 of element 68 receives a supply of gas fromsource 88. In this mode the breathable gas mixture at the patient port18 is allowed to be exhausted to atmosphere through port 24 in valve 22.However, the continuous presence of a back pressure from surce 88 asapplied through restrictors 96, 98 and tube 64 to control chamber 26assures a continuous positive airway pressure (CPAP) at the patient port18. This mode is considered beneficial in treatment of diseases such ashyaline membrane disease.

When the gas supply source 88 as well as source 90 is removed by closingvariable restrictor 96, the respirator is still functional and maintainsa constant flow of gas past the patient port 18. This mode may, forexample, be used to deliver a specified concentration of oxygen to apatient who does not require other respiratory support.

Having thus described a respirator in accordance with the invention, itsmany advantages can be appreciated. Gas is constantly flowed past thepatient who thus may inspire at any time. The respirator may be formedwith relatively inexpensive components and is sufficiently light inweight to render it portable. The variety of operating modes provide aversatile respirator suitable for treatment of many different diseasestates. The use of pneumatic controls provides a respirator which may beused in explosive environments.

What is claimed is: .[.1. A compact respirator operating in an ambientenvironment comprising .Iaddend.
 5. The respirator as claimed in claim 4.[.wherein said gas source producing means further includes.]. .Iadd.andfurther including .Iaddend.means for interrupting the flow of gas fromthe source to the maximum pressure control while enabling gas flow tothe minimum pressure control to provide a continuous pressure to the.[.diaphragm.]. .Iadd.control valve .Iaddend.and establish a continuouspositive airway pressure at the patient port.
 6. The respirator asclaimed in claim 4 wherein the pneumatic oscillating means furtherincludes .Iadd.a pair of pneumatically operated bistable elementscoupled in feed-back relationship to provide a pneumatic oscillator anda pneumatic delay element operatively interconnecting the pair ofbistable elements to provide a time constant with the inspiration andexpiration time control for control of the pneumatic oscillator;.Iaddend. .[.an.]. inspiration time control .Iadd.means .Iaddend.operatively responsive to the gas source .[.producing means.]. toprovide an inspiration pressure control pulse .[.of.]. .Iadd.through oneof the pair of feed-back coupled bistable elements to the pneumaticdelay element .Iaddend.a magnitude selected to establish the duration ofthe maximum inspiratory pressure level at the patient port; and .[.an.].expiration time control .Iadd.means .Iaddend.operatively responsive tothe gas source .[.producing means.]. to provide an expiratory pressurecontrol pulse .[.of.]. .Iadd.through said one bistable element in thepair of feed-back coupled bistable elements to the pneumatic delayelement with .Iaddend.a magnitude selected to establish the duration ofthe desired expiratory pressure level. .[.7. The respirator as claimedin claim 6 werein the pneumatic oscillating means further includes apair of pneumatically operated bistable elements coupled in feed-backrelationship to provide a pneumatic oscillator and a pneumatic delayelement operatively interconnecting the pair of bistable elements toprovide a time constant with the inspiration and expiration time controlfor control of the pneumatic oscillator..]. .[.8. The respirator asclaimed in claim 7 wherein one of the pair of pneumatic bistableelements is coupled to drive the bistable gas pressure controllingelement and the other of the pair of pneumatic bistable elements iscoupled to the inspiration and expiration time controls..].
 9. .[.A.]..Iadd.In a .Iaddend.respirator for supplying a patient with breathablegas from a supply of gas .[.comprising.]. .Iadd.with .Iaddend..[.a..]. acontrol valve having an input port, an exhaust port, a control chamberand means operatively located between the control chamber and theexhaust port to controllably release breathable gas to atmosphere at theexhaust port during inspiration and expiration cycles of the respirator;.[.b..]. tubing network means, having a patient port, for operativelycoupling the supply of breathable gas to the input port of the controlvalve while enabling continuous breathing from the patient port; .[.c.pneumatic control means operatively coupled to the control chamber foralternately pressurizing the control chamber to levels whichrespectively establish inspiratory and expiratory breathing conditionsat the patient port in the tubing network means, said pneumatic controlmeans including.]..[.i..]. pressure control means for alternatelyproducing high and low pressure levels in said valve control chamberwith pressure levels being selected to determine the magnitude of theinspiratory and expiratory .Iadd.pressure .Iaddend.conditions at thepatient port, .[.ii..]. means for cycling the pressure control meansbetween its high and low pressure levels at a rate and for respectivedurations selected to enable the varying pressures in the valve controlchamber to establish desired inspiratory and expiratory .Iadd.pressure.Iaddend.conditions at the patient port in the tubing network means,.[.iii..]. first variable means coupled to the cycling means forpneumatically controlling the cycling periods of the cycling means andestablish correspondingly desired time periods for the inspiratory andexpiratory conditions at the patient port, .[.and.]. .Iadd.theimprovement comprising .Iaddend. .[.iv..]. second variable means.Iadd.effectively .Iaddend.coupled .[.to.]. .Iadd.through .Iaddend.thepressure control means .Iadd.to the control chamber in said controlvalve .Iaddend.for pneumatically selecting inspiratory and expiratorypressure levels in the control chamber of the control valve tocorrespondingly determine inspiratory and expiratory pressure levels ofthe gas supplied through the tubing network means to the patient. .[.10.A respirator for supplying breathable gas from a supply of gas to apatient port comprisinga. a control valve having an input port, anexhaust port and a control chamber and means for controllably releasingbreathable gas to atmosphere from the exhaust port b. tubing networkmeans having a patient port for coupling the supply of breathable gas tothe input port of the control valve while enabling continuous breathingfrom the patient port; c. pneumatic control means operatively coupled tothe control chamber of the control valve for selectively pressurizingthe control chamber to regulate the exhaust of breathable gas from theexhaust port and correspondingly and selectively establish inspiratoryand expiratory breathing conditions at the patient port in the tubingnetwork means, said pneumatic control means includingi. variablepneumatic oscillator means for repetetively producing a pair of timevariable pressure levels respectively corresponding to the time durationof the inspiratory and expiratory breathing conditions, and ii. meansincluding a bistable pneumatic logic element coupled to said oscillatingmeans to respond to the pair of time variable pressure levels producedin the variable pneumatic oscillator means and coupled to said supply ofgas for generating a first gas pressure to the control chamber selectedto enable an inspiratory state at the patient port with a first stablestate of the logic element and a second gas pressure to the controlchamber selected to enable an expiratory state at the patient port witha second stable state of the logic element..]. .[.11. The respirator asclaimed in claim 10 wherein said means including the bistable pneumaticlogic element further includes.]. .Iadd.A respirator for supplyingbreathable gas from a supply of gas to a patient port comprising a. acontrol valve having an input port, an exhaust port and a controlchamber and means for controllably releasing breathable gas toatmosphere from the exhaust port; b. tubing network means having apatient port for coupling the supply of breathable gas to the input portof the control valve while enabling continuous breathing from thepatient port; c. pneumatic control means operatively coupled to thecontrol chamber of the control valve for selectively pressurizing thecontrol chamber to regulate the exhaust of breathable gas from theexhaust port and correspondingly and selectively establish inspiratoryand expiratory breathing pressure conditions at the patient port in thetubing network means, said pneumatic control means includingi. variablepneumatic oscillator means for repetetively producing a pair of timevariable pressure levels respectively corresponding to the time durationof the inspiratory and expiratory breathing conditions, ii. .Iaddend.afirst variable restrictor selected to provide an output pressure .[.tocorrespond.]. .Iadd.corresponding .Iaddend.with a desired maximuminspiration pressure.[.;.]. .Iadd., .Iaddend..[.and.]. .Iadd.iii..Iaddend.a second variable restrictor selectd to provide an outputpressure .[.to correspond.]. .Iadd.corresponding .Iaddend.with a desiredminimum expiration pressure.[.;.]. .Iadd., and .Iaddend. iv. a bistablepneumatic logic element having a control port, input parts and an outputport, with its control port effectively coupled to said oscillatingmeans to respond to the pair of time variable pressure levels producedin the variable pneumatic oscillator means and with the input portseffectively coupled to said first and second variable restrictors togenerate at the output port a first gas pressure effectively coupled tothe control chamber and selected to enable an inspiratory pressurecondition at the patient port with a first stable state of the logicelement and to generate a second gas pressure to the control chamberselected to enable an expiratory pressure condition at the patient portwith a second stable state of the logic element.
 12. The respirator asclaimed in claim 11 wherein the pneumatic oscillator further includesapair of bistable pneumatic logic elements.Iadd., each having a controlport, input ports and an output port, with the output port of oneelement in the pair effectively .Iaddend.coupled in feedbackrelationship .Iadd.to the control input of the other element in thepair.Iaddend., one of said pair of logic elements .[.being.]..Iadd.having one input port .Iaddend.provided with a third variablerestrictor .[.selected.]. to provide .[.an output pressure.]. .Iadd.agas flow rate .Iaddend..[.whose.]. .Iadd.through said one element with a.Iaddend.magnitude .[.is.]. selected to determine the time perioddesired for the inspiration cycle and a fourth variable restrictor.[.selected to provide.]. .Iadd.effectively coupled to the other inputport of said one element, said fourth variable restrictor .Iaddend..[.anoutput pressure.]. .Iadd.providing a flow rate through said one elementwith a .Iaddend..[.whose.]. magnitude .[.is.]. selected to determine thetime period for the expiration cycle.
 13. In a respirator for supplyinga patient with .Iadd.a .Iaddend.breathable gas from a supply ofpressurized gas including patient communicating means for controllingthe flow of breathable gas from said supply and from said patientcommunicating means to atmosphere, the improvement comprisingfirst,second and third pneumatic elements each having an output port and apair of input ports and a control port to select pneumatic communicationbetween one input port and the output port; .Iadd.means forintercoupling said .Iaddend.first and second of said plurality ofpneumatic elements being .[.intercoupled,.]. with the output port ofeach said latter elements being connected to the control port of saidother of said first and second elements in positive feedbackrelationship to form a pneumatic oscillator, with the output port of thesecond pneumatic element providing a pneumatic output oscillatingbetween pressure levels corresponding to the inspiratory and expiratorycycles for the respirator; volume means coupled between the output portof said first pneumatic element and the control port of said secondpneumatic element; first variable means coupled between the supply ofgas and one input port of the first pneumatic oscillator element andsecond variable means coupling the other input port of the firstpneumatic element to atmosphere for respectively selecting the timeperiods for the inspiratory and expiratory cycles of the pneumaticoscillator; a third pneumatic control element having its control portresponsively communicating with the output port of the second pneumaticelement in the pneumatic oscillator and having its output port connectedto said means for controlling the flow of breathing gas; and thirdvariable means coupled between the input ports of the third pneumaticcontrol element and the supply of gas to select at the output port ofthe third pneumatic control element inspiratory and expiratory pressurelevels for breathable gas supplied to the patient.
 14. The respirator asclaimed in claim 13 wherein the second variable means includes avariable expiratory duration determining gas flow restrictor effectivelycoupled between one input port of the first pneumatic oscillator elementand ambient pressure, said first variable means further including avariable inspiratory duration determining gas flow restrictoreffectively coupled between the other input port of the first pneumaticoscillator element and the supply of pressurized gas.
 15. The respiratoras claimed in claim 14 wherein the third variable means includesavariable expiratory pressure determining gas flow restrictor and avariable inspiratory pressure determining gas flow restrictoreffectively coupled between said supply of pressurized gas and therespective input ports of the third control pneumatic element.